1. Field of the Invention
This invention relates to focusing devices for optical equipment and, more particularly, to a focusing device for optical equipment such as movie cameras and television cameras which have a picture-taking system or a range finder system.
2. Description of the Related Art
The invention relates to an optical device for a photographic camera, such as a video camera. When viewing an image an observer can easily distinguish between images from a film recording and those from a video camera, the optical impression of the film recordings generally being considered more pleasing. It is known that, apart from a lower resolution of the video image and an unsatisfactory contrast behavior compared with the film image, this effect is largely due to the considerable Depth of Field of the video image. On recording objects at a specific distance from the focal plane, in the case of a specific focal length and aperture, there is a fixed Depth of Field. It is generally the case that with increasing format size the Depth of Field decreases. Thus, compared with the ⅓ inch video format, the larger 35 mm film format has a Depth of Field lower by a factor of roughly five for the same diaphragm step and viewing angle.
In the case of digital image recording by a video camera, the image is recorded on an image recording device in the form of a photosensitive chip such as a CCD, which is small compared with a 35 mm recording format. Thus, the recorded video image has a much higher Depth of Field than the comparable film image. This characteristic can easily be perceived by the image viewer. The considerable Depth of Field of a video camera is undesirable for photographing purposes. Instead a limited Depth of Field is used as a design element, which is only possible to a very restricted extent with small recording formats.
To obtain the same Depth of Field as the 35 mm film camera with the same viewing angle and aperture, a Depth of Field adapter is placed in front of the video camera. The main function of the Depth of Field adapter is to take the image of the still camera lens or movie camera lens and project it on the focusing screen (also known as ground glass, or GG). The video camera then will capture the image on the focusing screen. Still camera lens or movie camera lens (such as a Nikon lens) is mounted to the adapter using a lens mount. The function of the lens mount is to keep the lens at a proper distance from the focusing screen so it will project sharp image on the focusing screen. This distance is usually called Flange Focal Distance, or FFD.
The focusing screen of the 35 mm is usually moving to hide the stationary grains of the focusing screen. The movement is usually by vibrating, oscillating or spinning. In a conventional spinning 35 mm adapter, the center of a round focusing screen is mounted onto the shaft of a motor. When the shaft of the motor spin, the focusing screen will spin with it hence hiding the stationary grains of the focusing screen. The problem with this method of mounting the focusing screen onto the shaft of a motor is that when the shaft of the motor wobbles, which all do, it will cause the focusing screen to wobble. When the focusing screen wobbles, the Flange Focal Distance will be constantly changing, thus making the image constantly going in and out of focus, meaning the image is less sharp. Another problem with this mounting method is that since the motor is normally big, it will block the view of the frame and to compensate for the blocked view, the focusing screen must be made bigger leading to a bigger overall design of the Depth of Field adapter.
Another problem that all Depth of Field adapters face is that due to manufacturing errors, change in temperature, the Flange Focal Distance of a lens changes, thus the projected image on the focusing screen is not sharp at a given distance. To overcome this, usually paper shims or adjustable screws on the lens mount are used to set the lens mount at a proper distance to achieve the proper Flange Focal Distance.
Yet another problem that all Depth of Field adapters face is that often times the image sensor of the video camera is offset from the center of the lens, and when the Depth of Field adapter is attached to the camera, the image on the focusing screen of the Depth of Field adapter is also shifted from the center. A normal fix for this is making the connection ring between the Depth of Field adapter and the camera lens a little loose to allow the whole adapter to shift and use the screws to hold it in place at proper center. This causes the connection between the Depth of Field adapter and the camera less stable.